Few forage species require the level of management needed for alfalfa. From field establishment to the feedbunk, alfalfa growers need to be on top of their game to realize the crop’s potential.
Perhaps there is nothing more critical to successful alfalfa production than an optimum soil pH, and this needs to be addressed long before seed is ever purchased.
It’s not surprising that some of the best alfalfa-growing regions in the U.S. are those where the native soil pH is 6.8 or higher. Often, such soils are derived from a limestone base. Of course, that’s not the case everywhere. Many regions have naturally acidic soils.
Why doesn’t alfalfa grow where soils are acidic, even if fertility is high?
The major hindrance to alfalfa production where a low soil pH exists is aluminum toxicity. In acidic soils and subsoils, aluminum becomes soluble and is made more available for plant uptake.
Too much available aluminum has the effect of slowing and pruning alfalfa root growth. As a result, it reduces the activity and ability of Rhizobia bacteria to inhabit root nodules and fix nitrogen for the plant. This is why alfalfa looks nitrogen deficient where soil pH is too low.
Aluminum toxicity is best mitigated by correcting an acidic soil with an application of agricultural lime — preferably six months to a year before an alfalfa seeding is made. Correcting the soil pH also enhances the availability of other macronutrients such as phosphorus and potassium; however, applying lime adds a significant input cost and will likely need to be repeated over years as soils revert to their native acidic pH level.
More is needed
Research has shown that even with a lime application, subsoil pH can remain low and aluminum toxicity will still temper production potential. To resolve this situation, gypsum applications may help.
Gypsum, which is calcium sulfate, won’t raise the soil pH, but the high volume of calcium will displace the aluminum on subsoil particles and allow it to be leached below the rooting depths when enough moisture is available.
Research from the University of Georgia showed that alfalfa root growth increased 200% at a depth of 2.5 feet where gypsum had been applied. A subsoil (deeper than 15 inches) soil test is recommended to confirm if a gypsum application is warranted to reduce potential aluminum toxicity issues from low pH and high aluminum availability. Toxicity is likely when the subsoil pH trends below 5.5.
To ensure proper root growth, alfalfa needs moisture, an abundance of calcium at the root tip, and an absence of aluminum. The sulfate in the gypsum binds and precipitates out the aluminum. Coupled with the added calcium, this enhances the subsoil rooting environment, especially in fields where soil pH has been low for an extended period.
Gypsum — applied alone — will have no impact on soil pH. It must be used in conjunction with agricultural lime and a recommended fertilizer program. A well-drained site is also important for sustained alfalfa production, which is true regardless of the native soil pH.
Another means to mitigating the low soil and subsoil pH problem is to develop alfalfa varieties with tolerance to aluminum toxicity. Some plant breeding programs at land-grant universities in the Southern states are currently pursuing this approach.
